linux-kernel - Re: [PATCH] perf report: Do not process non-JIT BPF ksymbol events

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Message-ID: <Z8lEdWxt8CKepTJ3@google.com>
Date: Wed, 5 Mar 2025 22:45:09 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org, bpf@...r.kernel.org,
	Kevin Nomura <nomurak@...gle.com>, Song Liu <song@...nel.org>
Subject: Re: [PATCH] perf report: Do not process non-JIT BPF ksymbol events

Hello,

On Thu, Mar 06, 2025 at 08:25:01AM +0200, Adrian Hunter wrote:
> On 6/03/25 01:28, Namhyung Kim wrote:
> > The length of PERF_RECORD_KSYMBOL for BPF is a size of JITed code so
> > it'd be 0 when it's not JITed.  The ksymbol is needed to symbolize the
> > code when it gets samples in the region but non-JITed code cannot get
> > samples.  Thus it'd be ok to ignore them.
> > 
> > Actually it caused a performance issue in the perf tools on old ARM
> > kernels where it can refuse to JIT some BPF codes.  It ended up
> > splitting the existing kernel map (kallsyms).  And later lookup for a
> > kernel symbol would create a new kernel map from kallsyms and then
> > split it again and again. :(
> > 
> > Probably there's a bug in the kernel map/symbol handling in perf tools.
> > But I think we need to fix this anyway.
> > 
> > Reported-by: Kevin Nomura <nomurak@...gle.com>
> > Cc: Song Liu <song@...nel.org>
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  tools/perf/util/machine.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index 3f1faf94198dbe56..c7d27384f0736408 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -779,6 +779,10 @@ int machine__process_ksymbol(struct machine *machine __maybe_unused,
> >  	if (dump_trace)
> >  		perf_event__fprintf_ksymbol(event, stdout);
> >  
> > +	/* no need to process non-JIT BPF as it cannot get samples */
> > +	if (event->ksymbol.len == 0)
> > +		return 0;
> 
> Are all ksymbol events BPF?  Maybe it is OK
> for PERF_RECORD_KSYMBOL_TYPE_OOL also.  Perhaps adjust the
> comment in that case.

Probably, but I didn't see OOL with zero length yet.  Is it possible?

Thanks,
Namhyung